Siamese Colour Genetics Course · Lesson 2 of 8
Every Siamese kitten I’ve ever bred has arrived the same way: a small, pale, almost-white scrap with barely a hint of colour on it. The dark ears, mask, legs and tail that make a Siamese instantly recognisable simply aren’t there at birth — they develop over the first weeks of life. So why are Siamese cats pointed at all, and why do the points appear when they do? The answer is one of the most elegant tricks in the whole of cat genetics, and once it clicks, a great deal about your kittens starts to make sense.
In Lesson 1 we covered how colour is inherited in general — dominant, recessive, and what “carrying” means. This lesson is about the single gene that creates the Siamese pattern itself: the colourpoint gene. It sits on top of everything else. Whatever base colour, dilution or pattern a cat inherits, it’s this gene that decides the colour only shows on the cooler parts of the body.
In this lesson
- Why a Siamese’s colour appears only on its points, and never across the body
- How one temperature-sensitive gene creates the entire pointed pattern
- Why every Siamese kitten is born almost white
- Why points darken with age — and what a shaved patch tells you
- What it means to “carry” the colourpoint gene without showing it
What “pointed” actually means
The “points” are the ears, the mask (the face), the legs and feet, and the tail. On a Siamese these carry the colour, while the body stays pale — ivory, cream or a soft fawn depending on the cat. It’s worth being clear about one thing from the start: pointing is not a colour in its own right. A seal point isn’t “seal-coloured all over with a pale bit in the middle.” The cat is genetically seal from nose to tail — but the colour is only allowed to develop on the points. Everything else about the colour, the seal or the blue or the chocolate, is decided by other genes we cover in later lessons. This gene decides only where the colour is allowed to show.
The colourpoint gene — the C locus and the cs allele
Coat colour depends on an enzyme called tyrosinase, which the body uses to manufacture pigment. The gene that controls tyrosinase sits at what geneticists call the C locus. The normal, full-colour version of this gene — written C — makes tyrosinase that works properly everywhere on the cat, giving an evenly coloured coat.
Siamese carry a mutated version of that gene, the cs allele. It’s recessive, so a true Siamese needs two copies — cs/cs — for the pattern to appear. What the cs version produces is a slightly faulty tyrosinase: an enzyme that only works properly when it’s cool, and switches off when it’s warm. That single quirk — a temperature-sensitive enzyme — is the whole Siamese pattern in a nutshell.
Why temperature is the switch
A cat’s body core is warm, sitting a little under 38.5°C. The extremities — the ears, the nose and mask, the feet, the tail — run a couple of degrees cooler, because they’re further from the core and lose heat more easily. In a full-colour cat this makes no difference: tyrosinase works everywhere, so the whole coat is coloured. In a Siamese it makes all the difference. The cs enzyme is active in the cool extremities, so pigment develops there and the points come out dark. Across the warm torso, the same enzyme is switched off, so almost no pigment is laid down and the body stays pale.
Geneticists call this temperature-sensitive partial albinism. “Albinism” because the gene reduces pigment; “partial” because it doesn’t remove it entirely; and “temperature-sensitive” because heat is the control. The Siamese is, in effect, a cat wearing a thermal map of its own body.
Why kittens are born almost white
This is where the mechanism explains something every Siamese breeder sees first-hand. Inside the womb, the whole kitten is held at the mother’s body temperature — evenly, warmly, all over. There’s no cool ear or cool tail-tip in there. So the cs enzyme is switched off everywhere, no pigment develops, and the kitten is born pale from nose to tail.
The moment a kitten is born into cooler air, its extremities begin to cool below body core. The enzyme in those cooler spots wakes up, pigment starts to be produced, and over the following days and weeks the points darken into place — nose and ears first, then the mask, legs and tail filling in. It’s why you can’t reliably judge a newborn’s exact colour: a seal and a chocolate can look almost identical for the first fortnight, and only declare themselves as the points come through. As a GCCF judge I’ve handled a great many young Siamese, and the colour on a well-grown kitten tells you far more than the colour on a two-week-old ever could.
Points deepen with age — and with the weather
Because the pattern is driven by temperature, it never fully stops responding to it. Siamese points darken gradually throughout life, which is why an older seal point often carries a much richer, deeper coat than it did as a youngster, and why the body may shade darker with age too. Climate plays a part as well: the same cat kept in a warm home tends to hold a paler body than it would in a cold one, and cats living in cooler conditions often colour up more heavily. None of this changes the cat’s genetics — it’s the same gene responding to the same temperature rule, year after year.
The shaved patch that grows back dark
Here’s the gene demonstrating itself in real time. If a Siamese has a patch of fur clipped — for surgery, or for a blood sample — the newly exposed skin sits cooler than it did under a full coat. When the fur grows back, it frequently comes in noticeably darker than the surrounding coat, because the enzyme was more active over that cooler patch while the new hairs were forming. Over the following months, as the coat returns to normal and the skin warms back up, the dark patch usually fades and blends in. It’s harmless, and it’s the clearest everyday proof you’ll ever see that temperature, not some fixed “colour map,” is what paints a Siamese.
Blue eyes — the same gene at work
The cs allele doesn’t only affect the coat. It also reduces pigment in the iris, which is why every true pointed Siamese has blue eyes. The blue isn’t a separate “blue-eye gene” bolted on — it’s the same partial-albinism mechanism reducing pigment in the eye as it does across the body. It’s a useful sanity check, too: solid, pale-bodied points with clear blue eyes are the Siamese signature.
What one copy does — carriers of pointing
Because cs is recessive, a cat needs two copies to be pointed. A cat with one full-colour copy and one colourpoint copy — C/cs — looks like an ordinary full-coloured cat, but it carries pointing, exactly the way a seal can carry chocolate. This matters most at the edges of the breed and in related breeds: the colourpoint allele turns up in Orientals and other cats in the Siamese group, so two full-coloured carriers mated together can produce pointed kittens seemingly “out of nowhere.” Within pure Siamese breeding it’s simpler — both parents are cs/cs, so every kitten inherits two copies and every kitten is pointed. The pattern is guaranteed; only the colour on the points is up for grabs.
What this means at the nest
Put it together and the practical picture is straightforward. Mate a Siamese to a Siamese and you will get pointed kittens, every time — because both parents supply the cs allele. What you’re really predicting when you plan a Siamese litter isn’t whether the kittens are pointed, but what colour those points will be: seal, blue, chocolate, lilac, red, tortie, tabby and the rest. That’s the job of the genes we tackle from here on. The colourpoint gene sets the stage; the base-colour gene chooses the first paint.
Key takeaways
- The Siamese pattern comes from the recessive cs allele at the C locus — a true Siamese is cs/cs.
- The cs enzyme only works in cool areas, so colour develops on the cooler points while the warm body stays pale.
- Kittens are born almost white because the womb is uniformly warm; the points come through in cooler air over the first weeks.
- Points deepen with age and in cooler conditions — the same gene responding to the same temperature rule.
- Blue eyes come from the very same gene. Siamese × Siamese always produces pointed kittens; only the point colour varies.
In Lesson 3 we start on the colours themselves, beginning with the base-colour gene — the one that decides whether those points come out seal, chocolate or cinnamon, and why a seal so often has a hidden colour tucked underneath.
